[sjruckle]

Powder? https://dan-ball.jp/en/javagame/dust2/

[gpmcadam]

Very similar to this! I wonder if someone else had re-made it since then.

[tomjakubowski]

Check out sandspiel: https://sandspiel.club/

[iam4722202468]

Here's another powder simulation, https://powdertoy.co.uk/. This one still has an active community and development team, and people have built all kinds of cool things with it like actual computers that can be programmed with assembly.

[jms55]

The aforementioned sandspiel is excellent, along with orb.farm (by the same author), and (shameless plug) my own entry in the genre: https://github.com/JMS55/sandbox